The present invention relates generally to a transfer unit of an image forming apparatus and particularly to a transfer unit provided in an image forming apparatus such as a full-color copying machine.
In the full-color copying machine, a transfer drum is generally provided adjacent to a photoconductor drum. Printing paper is wound on the outer surface of the transfer drum, wherein an image formed on the photoconductor drum is transferred onto the printing paper on the transfer drum when pressed against the photoconductor drum.
In the case of color image formation processing onto large-size printing paper wound on such a transfer drum, it is impractical to transfer images of respective colors for each rotation of the drum, since the transfer drum rotates relatively rapidly compared with the operation speeds of the associated components. Therefore, when an image is to be transferred onto large-size printing paper wound on the transfer drum, each time an image of one color has been transferred onto the paper, the transfer drum is run in an idle state for one rotation, so that the respective components may be returned to their initial positions during the idle running of the drum. An image of another color is then transferred onto the paper during the next rotation of the drum. The transfer drum is separated slightly from the photoconductor drum while it is running idle so that toner remaining on the photoconductor drum does not adhere to the printing paper wound on the transfer drum. While the transfer drum is separated from the photoconductor drum, gears coupling these drums remain engaged with each other.
In order that the transfer drum be pressed against or separated from the photoconductor drum, it is supported by a rotatable frame. This frame is impelled toward the photoconductor drum through the agency of a pressing spring, whereby the transfer drum is pressed against the photoconductor drum during a copying operation. When the transfer drum is to run idle, the frame is rotated in the direction away from the photoconductor drum and in opposition to the spring by means of a cam provided in the frame toward the rear of the machine. As a result, a predetermined gap is given between the transfer drum and the photoconductor drum.
In order to dismount the transfer drum from the copying machine body in case of malfunction such as a paper jam, the pressing spring is disengaged, whereby the transfer drum may be detached from the photoconductor drum. Only while the two drums thus are not in contact with each other can the transfer drum be removed from the copying machine body.
In the structure of a conventional machine as described above, the frame supporting the transfer drum is pushed toward the photoconductor drum by means of the spring. The transfer drum is separated from the photoconductor drum by means of the cam. In order to effect this separation the cam presses only one point of a rear portion of the transfer unit. Consequently the frame is distorted, creating between the drums a larger gap toward the rear of the unit, where the cam is provided, than toward the front. Then if an adequate gap is to be provided toward the front of the unit, the gap resulting toward the rear will be considerably large, resulting in increased backlash from the transmission gears provided in the two drums, and causing deviations in the color images or other effects unfavorable to the full-color copying process.
Particularly, the structure of the conventional machine is such that it is necessary to impel the transfer drum and the frame supporting it toward the photoconductor drum by means of a pressing spring, which accordingly must have a considerably large coefficient of elasticity. Consequently, when the transfer drum is separated from the photoconductor drum by such a force as to overcome the elasticity of the spring, the frame is subject to considerable distortion.
The frame supporting the transfer drum has side walls on front and rear of the transfer drum. A guide path for guiding paper to the transfer drum is disposed between the front and rear side walls. A sheet fed to the transfer drum through the guide path is nipped by a clip provided in the transfer drum, whereby it is wound onto the transfer drum.
If the clip fails to nip the printing sheet, the sheet halts in the guide path, leading to a paper jam. In this state, when the transfer drum is drawn out in the axial direction of the drum in order to remedy the jam, the leading edge of the jammed printing sheet may catch on the rear side wall of the frame and become torn, or the printing sheet when caught on the rear side wall, may impede the transfer drum from being drawn out further, making it difficult to handle the paper jam.